Conventional packing cases for frozen shrimps or lobsters have the structure shown in FIG. 1 and 2, having a rectangular case body 1 for containing a proper volume of water and shrimps (not shown in Figures) and a plastic film 11 glued to seal the case body 1.
In the course of freezing the conventional packing case, the upper central section of the case can bulge up owing to the inflating pressure produced in the course of freezing, because the central section is the slowest in freezing. Then the plastic film has its central section bulging up as FIG. 2 shows. In other words, the packing case is deformed after frozen, losing its balanced outward shape and being very inconvenient for piling up in transportation.